Passive night vision devices, based on micro-channel image intensifiers, are capable of up to a million-fold multiplication of available light. The phosphor screen output of such devices can be directly coupled or imaged onto a video camera for viewing on a TV monitor.
However, passive night vision devices have severe limitations. Frequently, the amount of ambient light is insufficient for proper target illumination, and targets cannot be identified, especially at long ranges. In that event, active illumination is required. U.S. Pat. No. 4,642,452 to Loy discloses an integrated active night vision system consisting of a low power near infrared illumination source, an intensified receiver and a monitor. Illumination is provided by low power light flashes of very short duration. The light source is either a diode laser which emits pulses on the order of a microsecond in the near infrared, or a flashlamp with output pulses of 10 to 20 milliseconds duration which are passed through an infrared filter. The image intensifier is gated to reduce atmospheric backscatter.
In situations where covertness is required, the active illumination source is restricted to the near infrared (IR) spectral region or below 0.4 .mu.m. A main disadvantages of operation within this spectral region is that the transmitted output radiation may not be eye safe between the transmitter and the target, precluding use of such systems for imaging applications where eye safety is of concern.
Lasers are a desirable light source for active imaging systems because the high brightness and coherence of laser beams permit illumination of targets over long distances up to about 10 km and more.
This invention provides an active imaging system based on a laser illuminating source operating in the infrared region which is invisible to the human eye, which is capable of penetrating the atmosphere over long distances, and which is safe to the human eye.